This invention relates to a starter motor used mainly for starting an engine of a vehicle.
FIG. 1 is a cross-sectional view of a portion of a conventional starter motor disclosed, for example, in Japanese Patent Application Unexamined Publication No. sho. 63-90665. This starter motor is of the coaxial type in which an armature rotation shaft 3 of a DC motor 2, an output rotation shaft 4 having a pinion 5 fixedly mounted on its front end (right end in the drawings), and an electromagnetic switch device (not shown) are disposed on a common axis. More specifically, the armature rotation shaft 3 is hollow, and a plunger rod 105 of the electromagnetic switch device disposed rearwardly of the DC motor 2 is extended into an internal passage 3a of the armature rotation shaft 3. The output rotation shaft 4 is received in a front end portion of the internal passage 3a, and the plunger rod 105 is abutted against the rear end face of the output rotation shaft 4 through a steel ball 106. Upon forward movement of the plunger rod 105, the output rotation shaft 4 is urged or pushed forwardly.
The output rotation shaft 4 has at its front end (right end) the pinion-5 meshingly engageable with a ring gear of the engine (not shown in the drawing), and the rear portion of the output rotation shaft 4 is inserted in the internal passage 3a of the armature rotation shaft 3. This inserted portion 4a is borne by a sleeve bearing 6 fixedly fitted in the internal passage 3a, so that the output rotation shaft 4 is slidable axially. Means for transmitting a drive force from the armature rotation shaft 3 of the DC motor 2 to the axially-slidable output rotation shaft 4 is constituted by a drive force transmitting device 8 including an overrunning clutch device (one-way clutch) 7. Interposed between the inner peripheral portion (disposed inwardly of the pinion teeth) of the pinion 5 and the output rotation shaft 4 is a pinion spring 10 which normally urges the pinion in a forward direction.
More specifically, the drive force transmitting device 8 comprises a planetary gear speed reducer 9 including a sun gear 9a formed on the outer periphery of the armature rotation shaft 3 as well as planetary gears 9b, and the above-mentioned one-way clutch device 7 which includes a clutch outer member 7a to which central support shafts 9c for the planetary gears 9b are fixedly secured, and a clutch inner member 7b having at its inner peripheral surface helical spline grooves 109d meshed with a helical spline portion 4c formed on an outer periphery of an enlarged-diameter portion 4b of the output rotation shaft 4. Reference numeral 11 denotes a pinion stopper.
In the above conventional starter device of the coaxial type, when it is intended to reduce the size of the DC motor, it is considered that this can be achieved by reducing the number of the teeth of the pinion to increase the reduction ratio between the ring gear and the pinion. In this case, in the conventional device, the pinion spring is provided between the inner peripheral portion T of the pinion and the output rotation shaft in radially overlapping relation, and therefore with the construction of FIG. 1, if the number of the teeth of the pinion is reduced, there is encountered a problem that the mechanical strength (i.e., the strength of the inner peripheral portion T of the pinion and the strength of the shaft) is reduced.
On the other hand, the sun gear 9a of the planetary gear speed reducer 9 is formed on the front end of the armature rotation shaft 3. The planetary gear speed reducer 9 comprises the sun gear 9a, an inner gear 107b formed on an inner peripheral surface of a front bracket 108, and the planetary gears 9b rotatably supported by the respective central support shafts 9c and meshed with the sun gear 9a and the inner gear 107b.
The central support shafts 9c of the planetary gear speed reducer 9 are fixedly secured to the clutch outer member 7a of the overrunning clutch device 7, so that a reduction output of the armature rotation shaft 3 can be transmitted to the overrunning clutch device 7. The clutch inner member 7b is provided inwardly of the clutch outer member 7a, and rollers 109c are provided between the clutch outer 7a and inner members 7b, and these parts constitute the overrunning clutch device 7. Formed in the inner peripheral surface of the clutch inner member 7b are the helical spline grooves 109d meshed with the helical spline portion 4c formed on the enlarged-diameter portion 4b of the output rotation shaft 4. A return spring 110 is provided between a stepped portion 109e on the front end of the clutch inner member 7b and the helical spline portion 4c, and urges the output rotation shaft 4 rearwardly. The front end of the clutch inner member 7b is supported by a bearing 111 fitted in the front bracket 108.
The pinion 5 is spline-connected to a straight spline formed on the front end of the output rotation shaft 4, and a forward movement of the pinion 5 is limited by the stopper 11. The pinion spring 10 is provided inwardly of the inner periphery of the pinion 5, and acts between the pinion 5 and a stepped portion 104b on the output rotation shaft 4, the pinion spring 10 urging the pinion 5 forwardly. The pinion spring 10 is provided in order to normally urge the pinion 5 forwardly after the stopper 11 is fixed in position and also to dampen an impact produced when the pinion 5 is abutted against the ring gear. The bearing 6 is mounted within the internal passage 3a of the armature rotation shaft 3, and supports the rear portion of the output rotation shaft 4.
In the coaxial-type starter motor of the above construction, the rotational drive force of the DC motor 2 is transmitted to the overrunning clutch device 7 via the planetary gear speed reducer 9, and is further transmitted to the output rotation shaft 4 spline-fitted in the clutch inner member 7b. At this time, the plunger rod 105 is driven forwardly to move the output rotation shaft 4 forwardly, so that the pinion 5 is brought into meshing engagement with the ring gear (not shown) of the engine, thereby starting the engine. After the engine starts, when the operator (driver) turns off the electromagnetic switch device, the plunger rod 105 is retracted, and the output rotation shaft 4 is returned to the original position (stationary position) under the influence of the return spring 110, so that the pinion 5 is disengaged from the ring gear. A reverse drive, applied from the engine immediately after the start of the engine, is prevented by the one-way clutch operation of the overrunning clutch device 7 from being transmitted to the DC motor 2.
The conventional starter motor is of the above construction, and the front portion of the output rotation shaft 4 is supported by the spline-fitting between the helical spline grooves 109d of the clutch inner member 7b and the helical spline portion 4c. However, with respect to the helical spline fitting, it is difficult to extremely reduce a clearance in the fitting portion from the viewpoint of ensuring a sliding ability. Therefore, a play of a certain amount is present between the output rotation shaft 4 and the clutch inner member 7b, and moreover since the fitting portion serving as the support portion is not provided at the front end portion of the clutch inner member 7b, the distance between this fitting portion and the pinion 5 is large, and hence a moment is large. As a result, because of such a play and such a large moment, the conventional starter motor has problems that abnormal noises are produced and that in the worst case, the output rotation shaft 4 is broken. Further, the helical spline grooves 109d not only receive the load but also serve as the sliding surface for the output rotation shaft 4, and therefore depending on the determined value of the clearance, this has often been a factor in an improper sliding of the output rotation shaft 4 because of deterioration of grease on the spline fitting portion and of the deposition of dust on this portion.